The present invention concerns a method for filtering a liquid metal, specifically aluminum, in which the first step consists of introducing the liquid metal into a treatment chamber, then cleaning and degassing the metal, and the second step consists of injecting the degassed metal into a porous filtration element.
It also concerns a device implementing the method, comprising a treatment chamber made of refractory material with a channel admitting the liquid metal to be filtered, an element for cleaning said metal, and an element for degassing the cleaned metal, said treatment chamber comprising a degassing chamber and a filtration chamber which are partially separated by a wall.
The demand for high quality metal elaboration is constantly increasing, necessitating the filtration of liquid metal in order to obtain a product free of impurities. Technical solutions based on current knowledge are meeting with limited success in terms of both product quality and economics. The various methods and devices for treating liquid metal in current use are operational only after several metal elaboration phases have taken place. These operations are both costly and time consuming, and they offer only linear effectiveness, expressed in terms of the percentage of inclusions eliminated.
Various procedures for treating liquid metal have been proposed, specifically, injecting gas using graphite rods, a rotor, graphite diffusers, or frontal filtration on a ceramic filter. The majority of industrial installations currently use a combination of two or even three of these techniques. These methods are expensive and none of them succeeds in completely filtering out inclusions.
Publication No. FR 2 669 041 discloses a device in the form of a case divided vertically into two compartments which communicate at the base of the case. In the method used with this device, the metal is degassed and stripped of oxides by a rotor which rotates in the first compartment, and then filtered through a ceramic filter disposed horizontally in the second compartment before being transferred to a receptor system.
The filter described in document GB 1 262 933 is designed to retain the impurities from a liquid metal in fusion, particularly aluminum and its alloys; it comprises a series of parallel tubular filtration elements connected to a filtration plate of particles resistant to molten aluminum and joined by premelted glass. This filter is located at the heater outlet so the metal passes through it before being molded and solidified into ingots.
In addition, European Publication No. EP 0 291 580 describes an in-line degassing and filtering device for molten metal with a receptacle divided into two chambers. The first chamber is for degassing the metal with two cylinders made of porous material located at the end of gas injection conduits, and the second chamber, which has an inlet defined by a plate made of porous material such as a ceramic, is designed for filtering the degassed metal.
Document GB 1 367 069 proposes a method for continuously eliminating metal constituents from liquid metal in a receptacle separated into two compartments communicating through a bed of granular refractory material. The metal is agitated in the first compartment using an agitation device angled at 45xc2x0.
In all these devices, as well as the device which is the object of U.S. Pat. No. 2,913,118, which is limited to use with alkaline electrolytic metals such as sodium, filtration is performed frontally and not tangentially, and the stream of metal to be purified is not separated into two distinct streams to allow thorough filtration.
The subject of the abstract of Japanese Patent Application No. 63 140070 is an apparatus designed for frontal filtration of fine impurities in a heated bath such as a galvanizing dip. This apparatus comprises a receptacle divided into two compartments by a ceramic filter in the form of a plate designed to retain the impurities circulating through the bath. This apparatus is merely a simple liquid filtration device which is not suitable for liquid metal.
The present invention proposes to overcome the disadvantages of the prior art with a method and a device implementing the method, which eliminate all operations or installations affecting metal quality located upstream of the filtration device, and which use only a single installation to achieve very high quality filtration of any liquid metal, whatever its melting point. This high quality filtration results from the fact that the liquid to be purified is separated into two distinct streams, one purified stream which is sent through the outlet canal of the device and one stream containing all the impurities, which is returned for a second treatment. This separation of the stream of cleaned, degreased metal is achieved by subjecting the liquid metal to pressure before filtration.
This goal is achieved by the method of the invention as described in the preamble and characterized in that the pressurized liquid metal is injected at high speed into the filtration element, tangentially to the surface of said element, while maintaining a flow of liquid on said surface to clean the surface while simultaneously filtering said metal; in that the filtered portion of the metal is evacuated after passing through said filter element, and in that the remaining, unfiltered portion of the metal is re-introduced into said treatment chamber for a second treatment.
It is advantageous to increase the effectiveness of cleaning the surface of the filtration element by varying the speed at which the liquid metal circulates in said element, or by creating a turbulence in the liquid metal at the surface of said filtration element.
The filtration device implementing the method of the invention, as defined in the preamble, is characterized in that it comprises at least one pump with an inlet terminal and a recirculating terminal, which injects the degassed metal tangentially into at least one porous filtration element located in an enclosure with an exit channel for the filtered metal, a means for reintroducing the non-filtered portion of liquid metal into the treatment chamber, and a means for regulating flow and pressure of the non-filtered metal.
Advantageously, the filtration element is located between the injection pump and the pressure and flow regulating means, with the upstream extremity of said element being connected to the recirculation terminal of said injection pump and its downstream extremity connected to said pressure and flow regulating means through a conduit leading to the wall of the treatment chamber which allows the non-filtered portion of metal to be reintroduced in preparation for a second treatment.
The cleaning element and the degassing element are preferably located in the degassing chamber, and the injection pump, the filtration element, and the pressure and flow regulators are located in the filtration chamber.
The admission terminal on the injection pump is advantageously located in the space defined by the base of the treatment chamber and the degassing element.
The size of the pores preferably varies depending upon the quality of metal filtration desired.
Depending upon the embodiment, the filtration element may be a tubular element comprising at least one longitudinal channel.
Preferably the filtration element, the injection pump, and the pressure and flow regulating means are made of either ceramic, graphite, or a porous refractory metal.